The coccidia comprise a diverse group of obligate intracellular protozoa of vertebrates, with a few species infecting invertebrate hosts. These parasites have been placed in the phylum Apicomplexa, characterized by the presence of an assemblage of organelles within the anterior end of the invasive stages. This apical complex facilitates the entry of the parasite into host cells. Most named species infecting vertebrates are homoxenous (one host in life cycle) or facultatively heteroxenous (may also have dormant merozoite or sporozoite stages capable of initiating an endogenous cycle) and most develop within epithelial cells of the intestine. Other species causing disease in man and domesticated animals are obligatory heteroxenous parasites, having an intestinal phase of development (asexual and sexual, or sexual only) in one host and extraintestinal development (usually asexual) in an intermediate host. The coccidia of medical and veterinary importance, the focus of this book, are found within three of the more than nine families comprising the true coccidia (suborder Eimeriorina).